This invention relates to a method and apparatus for drying wet particulate material. More particularly, the method and apparatus of this invention are directed to controlling the drying of particles in a bed in a drying oven so that the moisture content of the particles at the oven's product outlet is substantially constant, despite variations in the moisture of the particles as they enter or are placed in the oven.
In the manufacture of particulate solids such as cereal products, the wet particulate material is dried in an oven. It is desirable that the product at the oven's outlet has a uniform moisture content. Such a uniform moisture content is difficult to achieve when the particulate material varies in composition, particularly in moisture content, from day to day and even from hour to hour, as is the case in the food industry.
Many proposals have been made for varying one or more control parameters of the drying process in response to detected changes in a monitored parameter. In many cases, the mass flow of the particulate material or the temperature of the drying air is varied in accordance with deviations in the moisture content of the product at the output of the drying oven. In other cases, the drop in the temperature of the drying air upon passing through a bed of the particulate material is measured and used to control the drying process.
U.S. Pat. No. 3,367,038 to Bishop, Sr., discloses a system for drying rubber in which the monitored variable is a temperature differential between the temperature of the drying air at a preselected location and the temperature of the rubber in the vicinity of that location.
U.S. Pat. No. 3,259,995 to Powischill teaches a method and apparatus for drying fibers, chemicals and other materials, in which method and apparatus moisture content control is accomplished by continuously sensing the temperature of a drying medium upon passage of that medium through the material being dried, the sensing being relative to a predetermined theoretical temperature at the scanning point. The drying conditions and/or drying rate is then automatically varied in response to deviations in the temperature drop of the drying medium relative to a predetermined temperature drop at the scanning point.
As disclosed in U.S. Pat. No. 3,204,361 to Whitsel, Jr., the drying of wet particulate material as it travels through a drier includes a first phase in which the surface temperature of the particles rises, a second phase in which the surface temperature of the particles remains at a substantially constant value, and a third phase in which the particles again increase in surface temperature towards the temperature of the drier itself. Whitsel, Jr., is directed in particular to varying the heat input to the drier in response to movements of a temperature breaking point towards the inlet or outlet of the drier. The temperature breaking point is that point where the second drying phase ends and the third drying phase begins. Specifically Whitsel, Jr., discloses an apparatus having a series of thermocouples placed in a linear array about the location of the ideal temperature breaking point in the dryer. The thermocouples are connected in series to one another, the temperature of the drying medium being increased or decreased respectively in accordance with a decrease or increase in the total E.M.F. of the thermocouples.
An object of the present invention is to provide an improved method and apparatus for drying particulate material to obtain a uniform moisture content thereof despite variations in the moisture content of the particulate material at the onset of the drying operation.
Another, more particular, object of the present invention is to provide such a method and apparatus in which the production rate is held at a substantially constant level.